In the fabrication of semiconductor devices it is often desirable not only to form patterned conductive layers but to do so such that the conductive layer has a shape which enhances electrical contact. For example, rounded chip pads (i.e. "solder balls") enhance electrical contact with other metallization levels. Although there are many known methods for forming a patterned conductor layer on a substrate the two most common methods of forming such a layer are subtractive etching and lift-off techniques. In subtractive etching, after a blanket conductor layer is deposited on the substrate the layer is selectively etched in order to remove undesired portions thereof. In lift-off, a layer (typically an insulator such as a polyimide or a photoresist) is deposited on a substrate, and is patterned through a photomask. The conductive layer is then deposited on the patterned insulator and the insulator is removed from (i.e. "lifted off" of) the substrate, taking with it the undesired portions of the conductive layer. Of these two techniques, it has been found that lift-off is more desirable since the solvents used to remove the insulator in lift-off cause less damage to the underlying substrate than do the etch processes (e.g. a plasma etch or a reactive ion etch) used in subtractive etching. Also, the conductor profile resulting from lift-off processing minimizes step coverage problems in subsequent conductor layers.
Utilization of lift-off techniques is already a preferred method of forming patterned conductive layers and, thus, it would be of considerable advantage if these same techniques could be utilized in order to form patterned conductive layers having varied shapes, such as "solder balls". However, creation of such conductive layers would often require the use of more complex lift-off structures and multiple layers of imaged lift-off (polyimide) layers.
An exemplary lift-off process involves coating a substrate with a photosensitive polyimide, imagewise exposing and then developing the layer so as to expose selected portions of the substrate. Then a conductive material is applied to form a film across portions of the remaining polyimide layer as well as the exposed portions of the substrate. The remaining portion of the polyimide layer is then "lifted off" taking with it the undesired portions of the metal layer leaving only the desired pattern of conductive material on the substrate. See, for example, U.S. Pat. No. 5,006,488 issued to Previti-Kelly on Apr. 9, 1991, the contents of which are incorporated herein by reference. However, the process discussed in the Previti-Kelly '488 patent utilizes only a single polyimide layer and is mainly dedicated to solving problems associated with depositing a metallization layer at high temperatures and avoiding the use of protective barrier layers during photolithographic processing.
The use of multiple layers of polyimides with lift-off techniques was described in an article by Winter, "Metal Deposition with Polyimide Lift-off Technique", IBM Technical Disclosure Bulletin, Vol. 17, No. 5, October 1974, page 1309, in which a first layer of polyimide is patterned through a photoresist mask. After the metal is deposited, the photoresist mask is removed from the first polyimide layer and a second polyimide layer is applied for passivation.
U.S. Pat. No. 4,606,998 issued Clodgo et al. on Aug. 19, 1986 describes a method utilizing multiple polyimide layers in a lift-off structure compatible with high temperature metal deposition. In the Clodgo '998 patent, an initial polyimide layer is deposited upon the substrate followed by deposition of a second layer of high temperature polyimide upon the surface of the initial polyimide layer. Both polyimide layers are then heated to a temperature which is below the final curing temperature of the high temperature polyimide and above the final curing temperature of the initial polyimide layer. This will allow the high temperature polyimide layer to remain soluble in common developers while the initial polyimide layer, which is fully imidized, becomes substantially insoluble in common solvents. Photoresist is then applied, exposed and developed and the two polyimide layers are etched forming a via which exposes the substrate. Conductive material is then deposited across the device, namely upon the second polyimide layer and the exposed substrate. Finally, the high temperature polyimide layer (the second layer) is then lifted off taking with it the undesired portions of the conductive layer while leaving the patterned conductive layer and the initial polyimide layer which serves to passivate the conductive layer. Although multiple layers of polyimide are used with this process it is, like the others, directed towards solving the problem of providing a lift-off structure compatible with a high temperature metal deposition without the use of a barrier layer. These prior patents, although solving important problems related to the formation of patterned metallization layers using photosensitive polyimide films, do not deal with problems experienced when it is desirable to separately image and develop overlapping layers of photosensitive polyimide materials.
The problems experienced with creating overlapping non-congruous layers of photosensitive polyimide become readily apparent upon understanding the properties of negative acting resists. With negative acting photosensitive polyimide resists it is the unexposed (uncross-linked) portions of the layer which are initially removed to create the desired pattern. For example, when a negative acting photosensitive polyimide precursor is exposed to a predetermined pattern of radiation, such as UV light, the exposed portions undergo cross-linking, making them insoluble in a developer. Thus, upon application of the developer the unexposed (non-cross-linked) polyimides are selectively removed, creating a desired pattern for depositing a subsequent conductive film. Accordingly, imagewise exposing and developing multiple layers having incongruous patterns is problematic since activating energy passes through the second layer and causes cross-linking in the underlying (previously unexposed) polyimide layer. Thus, portions of the initial layer, which were intended to be removed by the developer, have now become insoluble due to exposure (cross-linking) to the activating energy. This has the effect of altering or destroying the desired pattern to be formed by the initial layer.
Thus, there exists a need for a method of forming multilayer lift-off structures and a method of forming a patterned conductive layer having complex configurations.